Nucleus – IGCSE Chemistry Definition
IGCSE Chemistry definition of nucleus: the dense central core of an atom containing protons and neutrons. Covers charge, mass, and size relative to the atom.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
The nucleus is the small, dense, positively charged centre of an atom. It contains virtually all of the atom’s mass but occupies only a tiny fraction of its volume. Understanding the nucleus is the starting point for atomic structure, which connects to every major topic in the 0620 syllabus.
The 0620 definition
The nucleus is the central part of an atom. It contains protons (positively charged) and neutrons (neutral), and is surrounded by electrons in shells.
Key facts
| Property | Detail |
|---|---|
| Charge | Positive (due to protons) |
| Mass | Contains nearly all the mass of the atom |
| Size | Very small compared to the overall atom |
| Composition | Protons and neutrons (collectively called nucleons) |
Charge of the nucleus
The nuclear charge = number of protons = atomic number
In a neutral atom, this positive charge is exactly balanced by the negative charge of the electrons. For example, a carbon nucleus has 6 protons (charge = +6) and is surrounded by 6 electrons (charge = -6), giving an overall charge of zero.
Mass of the nucleus
Since both protons and neutrons have a relative mass of 1, and electrons have negligible mass:
Mass of nucleus is approximately equal to the mass number (protons + neutrons)
The electrons contribute almost nothing to the total mass. An atom’s mass is essentially concentrated in its nucleus.
Size of the nucleus
The nucleus is extremely small compared to the whole atom. If an atom were the size of a football stadium, the nucleus would be roughly the size of a marble at the centre. Most of an atom is empty space where the electrons orbit.
Worked exam question
An atom of nitrogen has 7 protons and 7 neutrons. (a) State the charge on the nucleus. (1) (b) State the mass number of this atom. (1) (c) Explain why the atom is electrically neutral overall. (1)
Mark scheme
(a) +7 / 7+ [1]
(b) 14 (7 protons + 7 neutrons) [1]
(c) The number of protons (7, positive) equals the number of electrons (7, negative), so the charges cancel out [1]
Common exam mistakes
- Saying the nucleus contains electrons. Electrons are outside the nucleus, in electron shells. Only protons and neutrons are inside the nucleus.
- Confusing nuclear charge with mass number. The charge of the nucleus equals the number of protons only. Neutrons contribute to mass but not to charge.
- Forgetting that the nucleus is where essentially all the atom’s mass is concentrated. Electrons have negligible mass.
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